A pneumatic tire typically includes a pair of axially separated inextensible beads. A circumferentially disposed bead filler apex extends radially outward from each respective bead. At least one carcass ply extends between the two beads. The carcass ply has axially opposite end portions, each of which is turned up around a respective bead and secured thereto. Tread rubber and sidewall rubber are located axially and radially outward, respectively, of the carcass ply.
Unfortunately, the sidewalls of tires are susceptible to degradation and cracking from exposure to ultraviolet light and ozone in the air. This problem with sidewall cracking naturally become more pronounced as the tire ages over time and can become unsightly and ultimately of tire performance concerns.
To mitigate tire sidewall cracking waxes are frequently included in tire sidewall compounds. Over time such waxes migrate to the surface of the tire sidewall and in theory form a film which helps to prevent deterioration by ozone and/or ultraviolet light. However, the use of inappropriate waxes can result in the appearance of the tire side wall being damaged.
Antioxidants are also typically added to sidewall formulations to help prevent sidewall cracking caused by ozone in the environment. However, the use of even a high level of antidegredants is not a totally satisfactory solution to tire sidewall cracking because antidegredants dissipate over time and become less and less effective. Accordingly, antidegredants may not be effective over a long tire service life. In any case, JP2006-89612A discloses a rubber composition containing specified amounts of a rubber component, paraffin wax and a gelation agent which can be used in tire treads and sidewalls. JP11-181150A discloses a rubber composition which is suitably used for a tire tread or a sidewall and comprises a rubber component and wax comprising normal paraffin in an amount of 75 to 85% by weight and iso paraffin.
U.S. Pat. No. 6,837,288 discloses rubber blends of brominated isobutylene/para-methylstyrene copolymers of 9.5 to 20 weight percent aromatic monomer content and 0.2 to 1.0 mole percent benzylic bromine content. These blends are reported to offer good cure characteristics, good adhesion and flex crack resistance, as well as ozone resistance. These blends are touted as being useful in tire sidewalls and other applications.
U.S. Pat. No. 7,498,366 discloses a rubber composition for tire sidewalls which is reported to provide an excellent appearance and weather resistance. More specifically, this patent discloses a rubber composition for a tire sidewalls comprising 45 to 60 parts by weight of a carbon black, not more than 1.5 parts by weight of powder sulfur, 0.45 to 0.5 parts by weight of N-cyclohexyl-2-benzothiazolylsulfenamide and 1.5 parts by weight of paraffin wax in which a distribution of the number of carbon atoms is 20 to 50, a ratio (iso content/normal content) of iso content to normal content is 5/95 to 20/80, and in the distribution of the number of carbon atoms of each of the iso content and the normal content, a standard deviation/average number of carbon atoms of the iso content is 1.0 to 1.8 and a standard deviation/average number of carbon atoms of the normal content is 1.0 to 1.8 based on 100 parts by weight of a rubber component comprising a butadiene rubber in an amount of 35 to 55% by weight and a natural rubber.
U.S. Pat. No. 7,759,410 discloses a rubber composition which can be used in tire sidewalls to improve tear strength, flex crack growth resistance and weather resistance. This tire sidewall rubber composition is comprised of 15 to 120 parts by weight of a white filler, and 5 to 50 parts by weight of a graphite having an average particle diameter of 3 to 50 μm on the basis of 100 parts by weight of a rubber component comprising a natural rubber in an amount of not less than 30% by weight, wherein the rubber component comprises an epoxidized natural rubber in an amount of 10 to 70% by weight.
U.S. Pat. No. 8,592,609 relates to a method of making rubber compositions having improved crack resistance. This method involves: a mixing in a first step: i. a first polymer, ii. a filler, b. mixing in a second step: i. a second polymer, ii. a polyhydroxy compound, c. mixing in a third step: i. the mixtures obtained in steps (a) and (b), ii. optionally, additional filler, wherein said first polymer and second polymer may be the same or different, and wherein steps (a) and (b) can occur simultaneously or consecutively; wherein the filler or additional filler are not present in the second step.
U.S. Pat. No. 8,623,956 describes a rubber composition that can be used in tire sidewalls to provide both excellent fuel economy (low heat build-up) and high flex crack growth resistance while having processability excellent enough to eliminate the need for mastication. This rubber composition contains a modified natural rubber with a phosphorus content of 200 ppm or less; and carbon black and/or a white filler.
U.S. Pat. No. 8,952,098 reveals a rubber composition for a tire sidewall or base tread, comprising: a rubber component comprising at least two kinds of diene rubbers; and a myrcene polymer, wherein the rubber component comprises at least one kind of diene rubber selected from the group consisting of natural rubber, epoxidized natural rubber, and butadiene rubber, and the myrcene polymer has a weight average molecular weight of 1000 to 500000. This rubber composition rubber composition is depicted as offering good flex crack growth resistance, crack initiation resistance, durability, and fuel economy when used in tire sidewalls despite the use of a softener derived from non-petroleum resources.